The field of the invention is x-ray computed tomography (CT) and, in particular, the selection of CT system parameters for reducing radiation dose.
X-ray imaging exposes individuals to ionizing radiation while being imaged. This radiation dose has become an important concern for public health. While CT has played a dramatic role in the detection and staging of disease, subjects are at a small but increased risk of radiation-induced malignancy. The most frequently employed method to reduce radiation dose is to lower the x-ray tube current using subject size or weight-based protocols. Another important and related dose reduction technique is automatic exposure control (AEC), which involves the automatic adaptation of the tube current during the CT scan. That is, as the CT gantry rotates around the subject and as the subject travels through the gantry, the tube current is adjusted according to subject size to keep image noise and quality constant. While this widely-used approach can achieve a dose reduction of 40-50% without sacrificing image quality, there are still other considerations that could lead to further dose optimization.
Recently, there have been several physics and clinical studies on the use of lower tube potential, which is generally measured using peak-kilovoltage, denoted “kV” or “kVp”, CT imaging, with the purpose of improving image quality or further reducing radiation dose. A principle behind this is that iodine has increased attenuation, or CT contrast, at lower tube potentials than at higher tube potentials in the range of tube potentials available on CT scanners. In many CT exams using iodinated contrast media, to achieve the superior enhancement of iodine at lower tube potentials, improves the conspicuity of hypervascular or hypovascular pathologies owing to the differential distribution of iodine, for example, in renal and hepatic masses, and inflamed bowel segments.
Images obtained at lower tube potentials tend to be noisier, primarily due to the higher absorption of low-energy photons by the subject. Therefore, a tradeoff exists between image noise and contrast enhancement in determining the clinical value of lower tube potential. When subject size is above a particular threshold, the benefit of the improved contrast enhancement is negated by the increased noise level. In this situation, the lower tube potential may not generate better image quality than the higher tube potential for the same radiation dose. In other words, dose reduction may not be achieved with the lower tube potential because the higher tube potential is needed to maintain appropriate image quality. However, below this size threshold, various degrees of dose reduction or image quality improvement at the same dose can be achieved. Therefore, for a given subject size and clinical application, an optimal tube potential exists that yields the best image quality or the lowest radiation dose.
Existing clinical studies have used empirically-determined tube potentials for a certain patient group, with various levels of dose reduction or image quality improvement being observed. However, an exact knowledge of the dose-efficiency of different tube potentials to obtain a target image quality, and the dependence of the optimal tube potential on patient size and diagnostic task, remain to be quantitatively determined. In a more recent study, some have used a dose-normalized contrast to noise ratio (CNR) as the criterion to determine the optimal tube potential and to quantify its dependence on phantom sizes and contrast materials. Their results demonstrated that the selection of tube potential should to be adapted to the patient size and to the diagnostic task to a much higher degree than is common practice today in order to further reduce the radiation dose. However, clinical practices to prospectively select the optimal tube potential and determine the corresponding radiation dose level that takes into account both the patient size and the target image quality required by a particular diagnostic task are lacking.